<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .left{
            float:left;
            width: 40px;
            background: pink;
        }

        .right{
            /* overflow: hidden;
             */
             float:right;
            background: skyblue;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="left">123</div>
        <div class="right">321</div>
    </div>
</body>
</html>